The present invention relates to an optical head for a magneto-optical disk.
Recently, much attention has been is focused, as an erasable optical memory, on a magneto-optical disk. The substrate of the magneto-optical disk is made of glass or plastics. Especially, in view of productivity and easiness of handling, favorable consideration is given to plastics such as acrylic plastic, polycarbonate plastic, etc. However, these plastics are readily subjected to double refraction especially in the case of polycarbonate plastic in comparison with glass and therefore, have optical characteristics inferior to those of glass. Especially, in a disk made of plastic produced by injection molding, double refraction is likely to take place. Meanwhile, in the magneto-optical disk, signals are reproduced by using linearly polarized light. Thus, if double refraction takes place in the substrate of the magneto-optical disk, linearly polarized light is elliptically polarized, thereby resulting in excessive deterioration of the quality of the reproduced signals.
Namely, when signals are actually read from the magneto-optical disk, laser beams are usually transmitted through the substrate by using a lens having a numerical aperture (NA) of 0.5 to 0.6 so as to be condensed. When double refraction takes place in the substrate, linearly polarized light incident upon the substrate is elliptically polarized while being transmitted through the substrate. The above described double refraction is classified into two kinds, i.e. double refraction in the direction of the plane of the substrate and double refraction in a direction perpendicular to the plane of the substrate. It is to be noted here that the double refraction in the direction perpendicular to the plane of the substrate means that there is a difference between a refractive index in the direction perpendicular to the plane of the substrate and that in the direction of the plane of the substrate. Meanwhile, the double refraction in the the plane of the substrate means that there is a difference between refractive indexes in different directions in the plane of the substrate. The double refraction in the direction of the plane of the substrate equally affects polarized light at any position of a bundle of rays. On the other hand, effects of the double refraction in the direction perpendicular to the plane of the substrate, on polarized light varies according to positions in the bundle of rays. Namely, at the center of the bundle of rays, polarized light is incident upon the substrate in a direction approximately perpendicular to the plane of the substrate and therefore, is affected substantially only by the double refraction in the direction of the plane of the substrate so as to be hardly affected by the double refraction in the direction perpendicular to the plane of the substrate. However, since light at an outer portion of the bundle of rays is obliquely incident upon the substrate and thus, is affected by not only the double refraction in the direction of the plane of the substrate but also by the double refraction in the direction perpendicular to the plane of the substrate.
Meanwhile, in the case where an orientation of polarized light incident upon the substrate is parallel to or perpendicular to the plane of incidence, linearly polarized light is maintained as it is. A substrate produced by injection molding of polycarbonate plastic has a double refraction in the direction of the plane of the substrate on the order of 10.sup.-6 but has a double refraction in the direction perpendicular to the plane of the substrate on the order of 10.sup.-4 greater by two figures than the double refraction in the direction of the plane of the substrate. Hence, it follows that the change of linearly polarized light to elliptically polarized light is mainly caused by the double refraction in the direction perpendicular to the plane of the substrate.
It is supposed here that a Z-axis is directed in a direction perpendicular to the principal plane of the substrate and an X-axis, a Y-axis and the Z-axis have refractive indexes nx, ny and nz, respectively. Furthermore, assuming that character .alpha. denotes an angle formed between the plane of incidence of laser beams and the X-axis and character .theta. denotes an angle of refraction on the substrate, a refractive index ns in the direction perpendicular to the plane of incidence is approximately expressed by the following equation: EQU ns .perspectiveto.1/.sqroot.sin.sup.2 .alpha./(nx).sup.2 +cos.sup.2 .alpha./(ny).sup.2
where: .vertline.nx-ny.vertline..ltoreq..vertline.(nx+ny)/2-nz.vertline..
Meanwhile, a refractive index np in the direction of the plane of incidence is given by the following equation: EQU np.perspectiveto.1/.sqroot.cos.sup.2 .theta.(cos.sup.2 .alpha./(nx).sup.2 +sin.sup.2 .alpha./(ny).sup.2)+sin.sup.2 .theta.(nz).sup.2.
FIG. 1 shows how reflected light undergoes polarization at respective positions of a bundle of rays when linearly (polarized light polarized in the direction of the X-axis) is incident upon a prior art magneto-optical disk. FIG. 1A is a side elevational view showing a portion in the vicinity of the prior art magneto-optical disk and includes a condenser lens 1, a plastic substrate 2, a recording medium 3 and a light beam 4. FIG. 1B is a top plan view of FIG. 1A showing polarization of reflected light at respective positions of the bundle of rays. The linearly polarized light is maintained as it is on the X-axis and the Y-axis but gradually changes elliptically as the position of the bundle of rays is further outwardly displaced in directions forming an angle of 45 degrees with the X-axis and the Y-axis, thereby resulting in deterioration of the quality of the reproduced signals.